The present invention relates to an improved ,method for refurbishing an airfoil, such as a vane for a gas turbine engine, to restore a desired flow area.
A gas turbine engine includes a compressor section, a combustion section, and a turbine section. Disposed within the turbine section are alternating rows of rotatable blades and static vanes. As hot combustion gases pass through the turbine section, the blades are rotatably driven, turning a shaft and thereby providing shaft work for driving the compressor section and other auxiliary systems. The higher the gas temperature, the more work that can be extracted in the turbine section and the greater the overall efficiency. In order to increase the turbine section operating temperature capability, cobalt and nickel base superalloy materials are used to produce the turbine airfoil blades and vanes. Such materials maintain mechanical strength at high temperatures.
The static vanes, disposed between the rows of rotating blades, stabilize and direct the gas flow from one row of rotating turbine blades to the next row, with a nozzle area defined by the spacing between the adjacent vanes. Such gas flow stabilization optimizes the amount of work extracted in the turbine section. Generally, the nozzle flow area is assigned a series of classification numbers which are correlated to the volumetric gas flow. This allows comparison of flow properties between vanes of complex geometry. The nozzle area is therefore defined for convenience in terms of a class size. For example, in a particular model engine, a class 27 nozzle has an open area of 1.868-1.894 square inches, while a class 29 nozzle has an open area of 1.919-1.944 square inches, regardless of geometry.
In service, deterioration of the vane surface(s) occurs due to oxidation and metal erosion caused by abrasives and corrosives in the flowing gas stream impinging on the vane. In addition, high gas loadings at high temperature promote distortion of the vanes, thereby enlarging the nozzle area, with a consequent loss in turbine efficiency. During a periodic engine overhaul, the vanes are inspected for physical damage and evaluated to determine the degree of flow area change and the effect on nozzle classification. Before such vanes can be returned to the engine, any eroded material must be replaced and the vanes otherwise reclassified. In addition, any vanes which suffer a loss of metal or a change in shape due to coating removal or repair must be reclassified.
Several methods exist for modifying a vane to return the nozzle gas flow area to the original classification (reclassifying). One method involves hot striking or otherwise bending the trailing edge of the vane, narrowing the gap between adjacent vanes. However, such bending introduces stresses which may produce cracks in the vane. Such bending may also cause excessive distortion of the vane, preventing the proper fit and seal of the internal cooling tubes, while the fixturing devices, which hold the vanes during bending, may distort the vane platform or crush the vane pedestal. Even if bending stresses can be reduced, several high temperature alloys used in gas turbine engines cannot be hot formed or bent due to the deleterious effects on material properties such as fatigue strength. Since the bending process does not return metal to the vane surface, there is no strength contribution and the vane is structurally weaker than a new vane would be, limiting the useful life of the vane.
Another method for reclassifying turbine vanes involves the addition of an alloy to the deteriorated vane surface by a combined weld/plasma spray process, such as that described in U.S. Pat. No. 4,028,787 to Cretella et al. This process requires the addition of weld beads to the worn surface for reinforcement, with a number of plasma sprayed layers of the alloy then added to achieve the proper alloy thickness. This procedure is very labor intensive requiring a welder to add a number of weld beads to a small surface, clean the vane, and then add a number of plasma spray layers. In addition, the vane may be damaged due to the thermal stresses involved in the welding operation.
Another problem with the weld/plasma spray process involves the specific area of deterioration. It is to be expected that deterioration will be more severe at the narrowest nozzle dimension where the velocity of the gas flow is highest. During the plasma spray process, alloy is added to the surface in very thin layers, forming a broad even pattern. After completion of the plasma spray, the excess material must be removed from non-eroded areas of the vane. If the deterioration is severe in specific areas, numerous layers of the alloy must be added and much of it removed from the non-eroded areas. Such a procedure is time consuming and wasteful of the ahoy materials involved.
Still another method for refurbishing gas turbine vanes is shown in U.S. Pat. No. 4,726,101 to Draghi et al. In this method, a build up of alloy in the wear area is accomplished by controllably applying layers of a tape of uniform thickness to the vane. The tape includes a mixture of a binder and an alloy powder, which is compatible with the substrate alloy, with the mixture formed into a sheet of uniform thickness and having an adhesive backing. After applying the tape in layers to a desired thickness, the vane is heated to a temperature at which the binder and adhesive decompose and the ahoy in the tape diffusion bonds with the substrate alloy.
Yet another method for refurbishing a gas turbine vane covered by a protective coating is shown in U.S. Pat. No. 5,142,778 to Smolinski et al. In this method, the protective coating is first removed from the surface of the vane. Thereafter, material is added to surfaces of the vane in the areas requiring repair or replacement of the eroded material and bonded to the surfaces. A laser beam is then directed at the surface in the distorted areas such that localized areas of the surface of the distorted areas are melted, solidified, and cooled to ambient temperature to form a recast layer. Any excess material is removed from the surface and the protective coating is reapplied.
The Draghi et al. and Smolinski et al. methods, while effective to refurbish vanes, are complicated to perform. Thus, there remains a need for a simpler approach for restoring a desired flow area of a gas turbine vane, which approach is also cost effective.